This document discusses creating objects from predefined classes in Java. It explains that a class name can be used as a type to declare an object reference variable, but no object is created yet. An object must be created separately using the new operator, which allocates memory and returns a reference to the newly created object. This reference is stored in the object reference variable. The document provides examples of creating String objects and invoking methods on objects using the dot operator. It distinguishes that primitive variables contain values directly, while object variables contain references to objects in memory. Assignment of object references copies the reference, not the object.

1. CREATING OBJECTS FROM
PREDEFINED CLASS
Chapter 6.3:

2. Creating Objects
 A variable holds either a primitive type or a
reference to an object
 A class name can be used as a type to declare an
object reference variable
String name;
 No object is created with this declaration
 An object reference variable holds the address of
an object
 The object itself must be created separately

3. Creating Objects
 Generally, we use the new operator to create an
object
 Creating an object is called instantiation
 An object is an instance of a particular class
name = new String (“Ali bin Ahmad");
This calls the String constructor, which is
a special method that sets up the object

4. Constructing String objects
Strings stringRef = new String(stringLiteral);
eg.
String name = new String(“Muhammad Haziq”);
Since strings are used frequently, Java provides a
shorthand notation for creating a string:
String name = "Muhammad Haziq”;

5. Constructing String objects
 New String objects are created whenever the
String constructors are used:
String name4 = new String(); // Creates an object
String name5 = new String("Socrates");
String name6 = name4;

6. Invoking Methods
 We've seen that once an object has been
instantiated, we can use the dot operator to invoke
its methods
name.length()
 A method may return a value, which can be used
in an assignment or expression
count = name.length();
S.o.p(“Num. of char in “ + name+ “=“ +
count);
 A method invocation can be thought of as asking
an object to perform a service

7. Object without object reference
cannot be accessed
String n1 = new String(“Ali“);
new String(“Abu“);
sv1: String
: String
value = “Ali”
value = “Abu”
n1
n1- object reference variable

8. Object References
 Primitive type variables ≠ object variables

9. References
 Note that a primitive variable contains the value
itself, but an object variable contains the address
of the object
 An object reference can be thought of as a pointer
to the location of the object
 Rather than dealing with arbitrary addresses, we
often depict a reference graphically
"Steve Jobs"name1
num1 38

10. Assignment Revisited
 The act of assignment takes a copy of a value and
stores it in a variable
 For primitive types:
num1 38
num2 96
Before:
num2 = num1;
num1 38
num2 38
After:

11. Object Reference
Assignment
 For object references, assignment copies the
address:
name2 = name1;
name1
name2
Before:
"Steve Jobs"
"Steve Austin"
name1
name2
After:
"Steve Jobs"

12. Questions
String stud1 = new String(“Ani”);
int studID = 65000;
What does variable stud1 contains?
What does variable studID contains?
Is this allowed? stud1 = studID;
String stud1;
stud1 = new String(“Ani”);
stud1 = new String(“Obi”);
How many objects were created by the program?
How many reference variables does the program contain?